A reliable, yet sensitive all-solid-state color video camera would find abundant utility including, for example, use in television, card readers, facsimile recorders, picturephones, and character recognition, etc.
Color photosensitive devices using charge-handling solid-state image sensors of various types, for example, charge-coupling devices, known as CCDs, and charge-coupling imagers known as CCIs, have been proposed for and used in video cameras. To avoid optical complexity and problems with image registration, it is highly desirable that color image sensing occur at a single imaging site, e.g., at a single planar photosensitive array. Difficulty is encountered with such "single-site" color imaging, however, because at least three distinct types of color information must be extracted in order to represent a color image in video signal form.
One known approach to providing a "single-site" color sensing device uses a single image sensor of broad wavelength sensitivity and a cooperating filter disc which passes a series of color filters through the image beam in a repeating sequence. The filter interpositions are synchronized to image scanning a filter typically being interposed during an entire field scan. Devices operating in this manner are said to produce a "field sequential" color signal. One problem with this approach is that the resulting signal presents the extracted color image information in a time order which is radically different from the time order of, for instance, the standard MTSC video signal. (The standard NTSC video signal is described in Chapter 16, "Television Transmission", of Transmission Systems for Communications, Revised Third Edition, by Members of the Technical Staff of Bell Telephone Laboratories, Copyright 1964, Bell Telephone Laboratories, Inc.) A further disadvantage is that some of the color image information (e.g., blue image information if a blue basic color is utilized) tends to be disproportionately detailed and hence wasteful of sensor capacity in consideration of the response characteristics of the human eye.
Certain other proposed approaches to achieving "single-site" color image sensing call for the use of striped color filters superimposed on a single image sensor. One such type of image sensor uses filter grids which are angularly superimposed on one another (see U.S. Pat. No. 3,378,633). As a result of image scanning, such image sensors produce a composite signal wherein chrominance information is represented in the form of modulated carrier signals. Such apparatus may be adapted to produce signals in the NTSC format or, if desired, the color image information can be separated by frequency domain techniques. In practice, however, it has proven difficult to produce such sensors economically, particularly where detailed image information is required.
Striped filters which transmit a repeating sequence of three or more spectral bands have also been proposed for use in color imaging. With this arrangement, the filters are typically aligned in one direction and scanning of the image is performed orthogonally to that direction. In effect, elemental sample areas are defined along the filter stripes. With this arrangement, it will be appreciated, sampling for a given color is not uniform for both directions. Additionally, the sampling patterns which result tend to provide a disproportionate quantity of information regarding basic color vectors to which the dye has less resolving power, e.g., "blue" information relative to "green" information.
Another approach to color imaging which has been proposed is the "dot" scanning system, as discussed in U.S. Pat. No. 2,683,769 to Banning. That approach generally uses spectrally selective sensor elements which are arranged in triads (red, green and blue elements, respectively). However, in U.S. Pat. No. 2,755,334, also to Banning, a repeated arrangement of four element groupings (red-, green-, blue-, and white-sensitive elements, respectively) is described. Such approaches to color imaging have not been of practical significance, in part because of the cost of fabricating the number of individual elements which are required to provide image information having adequate detail.
Many of the problems associated with the prior art discussed above are overcome by the approach taken in U.S. Pat. No. 3,971,065, issued July 20, 1976, in the name of B. E. Bayer. In the Bayer approach, color imaging is effected by a single imaging array composed of individual luminance and chrominance sensing elements that are distributed according to type (sensitivity) in repeating interlaid patterns, the luminance pattern exhibiting the highest frequency of occurrence--and therefore the highest frequency of image sampling--irrespective of direction across the array.
Preferably, to produce an element array according to the Bayer approach, a solid state sensor array of broad wavelength sensitivity is provided with a superposed filter array. Filters which are selectively transparent in the green region of the spectrum are preferably employed in producing luminance-type elements, and filters selectively transparent in the red and blue spectral regions, respectively, are preferably employed in producing chrominance-type elements. (The term "luminance" is herein used in a broad sense to refer to the color vector which is the major contributor of luminance information. The term "chrominance" refers to those color vectors other than the luminance color vectors which provide a basis for defining an image.)
Methods for providing multi-color filter arrays are known in the art. A particularly useful method is described in copending U.S. patent application Ser. No. 730,886, filed Oct. 8, 1976 in the name of A. T. Brault, W. A. Light, and Thomas W. Martin and entitled "A Method for Making a Solid-State Color Imaging Device Having an Integral Color Filter and the Device", now U.S. Pat. No. 4,081,277. The Brault et al. application describes a method for making a solid-state color imaging device that comprises coating a layer of a dye-receiving polymer on a solid-state photosensitive device and diffusing heat-transferable dyes into the polymer to produce a multicolor filter on the photosensitive device.
We have found that during the processing necessary to produce the color imaging devices described by Brault et al. supra, the dye-receiving polymer does not adhere well to the solid-state photosensitive devices, particularly silicon-based devices, and may be scraped off. Therefore, it would be desirable to find a material to promote the adherence of the dye-receiving polymer to the solid-state photosensor.